Blocked investigation

On 4 June last year Mr Range said "sufficient factual evidence exists that unknown members of the US intelligence services spied on the mobile phone of Chancellor Angela Merkel".

But in December he revealed that the investigation was not going well and he had not obtained enough evidence to succeed in court.

A statement from Mr Range's office on Friday said "the accusation cannot be proven in a legally sound way under criminal law".

It said "the vague statements by US officials about possible surveillance of the chancellor's mobile telecommunication by a US intelligence service - 'not any more' - are not enough to describe what happened''.

The prosecutor did not manage to obtain an original NSA document proving the alleged spying, and a transcript which purportedly re-created it from memory was deemed insufficient as evidence.

The BBC's Jenny Hill in Berlin says Germans are very concerned about privacy because of their own history. Mass surveillance was a characteristic of the Nazi era and the communist East German state.

But now President Obama and Chancellor Merkel have a fairly pragmatic, pleasant relationship, our correspondent says.

A German parliamentary committee is also investigating NSA surveillance, but it has not managed to get much help from US officials either, Germany's Spiegel news website reports.

The furore over US surveillance led to Germany's expulsion of a top CIA official from Berlin in July 2014.

Then in August German media reported that German intelligence had eavesdropped on phone calls by US Secretary of State John Kerry and his predecessor Hillary Clinton. Unnamed German government sources said the calls had been picked up accidentally.

Mr Snowden worked as a contractor for the NSA but fled to Hong Kong from the US in May 2013 after revealing extensive, global internet and phone surveillance by US intelligence. After speaking to The Guardian newspaper in Hong Kong he flew to Moscow, where he is still living.

The US charged him with theft of government property, unauthorised communication of national defence information and wilful communication of classified communications intelligence.